This invention relates to scintillators and scintillating materials. More particularly, this invention relates to a method for producing ceramic-like scintillator bodies comprising pressed phosphors, the scintillator bodies having high optical translucency with low light absorption and a high degree of physical integrity, which properties render bodies highly useful in computerized tomography detectors.
In general, a scintillator is a material which emits electromagnetic radiation in the visible or near visible spectrum when stimulated by high energy electromagnetic photons such as those in the x-ray and gamma-ray regions of the spectrum. Thus, these materials are excellent choices for use as detectors in industrial or medical x-ray or gamma-ray equipment. In most typical applications, the output from scintillator materials is made to impinge upon photoelectrically responsive materials in order to produce an electrical output signal in direct relation to the intensity of the initial X-ray or gamma-ray bombardment.
In general, it is desirable that the amount of output from these scintillators be as large as possible for a given amount of X-ray or gamma-ray energy. This is particularly true in the medical tomography area where it is desired that the energy intensity of the X-ray be as small as possible to minimize any danger to the patient.
As used herein and in the appended claims, the term "light" refers to electromagnetic radiation in the visible or near visible region of the spectrum so as to encompass those wavelength radiations in the infrared or ultraviolet regions typically emitted by the phosphors of interest herein. The term "optical" is also defined herein to include both the visible and these near visible wavelengths.
A scintillator body, in order to be effective, must be a good converter of high energy radiation (that is, X-rays and gamma-rays). Moreover, the scintillator body, to be efficient, must be a good transmitter of the light energy that results from high energy bombardment. If it is not, and this light is absorbed in the material, the overall conversion efficiency suffers. Thus, higher energy ionizing radiation must be applied to the scintillator to get the same electrical output from the overall system. For a medical tomographic X-ray system, this means poor quantum detection efficiency and a lower signal-to-noise ratio.
It is very desirable to grow single crystals of certain scintillator materials, however, the ability to grow such large crystals of these scintillating phosphors by conventional methods is not presently available. Hence, many scintillator materials that are in current use either have a polycrystalline or a powdered structure.
Another important property that scintillator materials should possess is that of a short afterglow or persistence. This means that there should be a relatively short period of time between the termination of the high energy radiation excitation and the cessation of input from the scintillator. If this is not the case, there is a resultant blurring, in time, of the information-bearing signal. Furthermore, if rapid scanning is desired, the presence of the afterglow can severely limit the scan rate, thereby rendering the viewing of moving bodily organs difficult.
Typical scintillator phosphors which have been used include barium fluorochloride doped with a europium activator (BaFCl:Eu). Other phosphors include lanthanum oxybromide doped with terbium (LaOBr-Tb), cesium iodide doped with thalium (CsI-Tl), calcium tungstate (CaWO.sub.4), and cadmium tungstate (CdWO.sub.4). If these phosphors and others are used in the powdered or polycrystalline form, the internal light path becomes extremely long and tortured, resulting in unnecessary absorption of light signal output. Many phosphors are not producible as single crystals which would have a greatly reduced amount of light absorption. In particular, attempts to grow BaFCl:Eu in single crystal form result in a material which is multifaceted and self-delaminating, like sheet mica. Thus, in these materials it is primarily the surface regions which contribute significantly to the light signal output resulting from high energy excitation.
Scintillator materials comprise a major portion of those devices used to detect the presence and intensity of incident high energy photons. Another commonly used detector is the high pressure noble gas ionization device. This other form of high energy photon detector typically contains a gas, such as xenon, at a high pressure (density), which ionizes to a certain extent when subjected to high energy X-ray or gamma-ray radiation. This ionization causes a certain amount of current to flow between the cathodes and the anodes of these detectors which are kept at a relatively high and opposite polarity from one another. The current that flows is sensed by some form of current-sensing circuit whose output is reflective of the intensity of the high energy radiation. Since this other form of detector operates on an ionization principle, after the termination of the irradiating energy, there still persists the possibility that a given ionization path remains open. Hence, these detectors are particularly characterized by their own form of "afterglow" which results in the blurring, in the time dimension, of the information contained in the irradiating signal.
Scintillation type detectors have several advantages over ionization type detectors used in tomographic applications. First, they do not require the maintenance of a gas at a high pressure (typically 25 atmospheres). Second, they are lower in cost. Third, they are easier to maintain. Fourth, they do not suffer from charge accumulation effects. Fifth, they are rigid and not sensitive to vibrational or microphonic noise sources as are the ionization detectors. Sixth, they are extremely rugged. Seventh, they do not require high voltages. Eighth, they can be easily coupled with photodiodes to form an entirely solid state device.